The cure rate of conventional single-component room temperature vulcanizable (RTV) formulations are limited by the rate at which atmospheric moisture diffuses into the curing formulation. One practical method by which the cure rate has been increased is to divide the formulation into two components, each of which are separately stable, but which upon mixing in the appropriate ratio, cures rapidly to produce a polymer network having the desired properties. Such two-component formulations are effective because the curing catalyst has been isolated in a first package from the hydroxy-terminated (silanol terminated) polymer which also contains an approximately stoichiometric quantity of water in the second package. When it is desired to prepare a room temperature vulcanizable silicone, mixing of the two components from the first and second packages initiates cure of the RTV silicone. This mode of packaging precludes the use of silanol terminated polymer in the catalyzed component and also limits the proportions in which two-component formulations may be prepared resulting in the need to use small quantities of the catalyst containing component relative to the polymer containing component.
These formulations are thus limited in their utility by the proportion of ingredients comprising each of the components. The first and major component of such two-part RTV formulations generally comprises a linear silanol polymer, both ends terminated by hydroxy (silanol) groups, and fillers. The second and minor component comprises crosslinking agents, adhesion promoters, plasticizing fluids and the cure catalyst. This results in the first component being mixed in a relatively high weight ratio of ten or fifteen to one relative to the weight of the second component.
There are several disadvantages associated with this disparate weight ratio of the two components. In automated continuous mixing equipment, long static mixers are required and there are difficulties associated with uniformly distributing the minor catalyst containing component in the larger polymer containing component. These difficulties are aggravated if the mixing is done by hand to produce small quantities.
Early work by Berridge, U.S. Pat. No. 2,843,555, showed that an RTV formulation comprising a hydroxy terminated silanol polymer, alkoxy-substituted silane crosslinking agents optionally containing mineral fillers remained stable and unchanged in viscosity until the formulation was intentionally cured by the addition of certain metal salts which catalyzed the self-condensation of the hydroxyl groups of the silanol or with the alkoxy groups of the crosslinking agent. More recently a composition has been disclosed that may be used either in one-component or two-component formulations (packaging). Fuijioka et al. in U.S. Pat. No. 5,300,611 disclose extending the minor catalyzed component by the addition of a trimethoxy endcapped silanol polymer. Because water is absent during storage, and because the crosslinker is compounded in the major component there is no reaction. Mixing the two components at a ten to one weight ratio yields a composition that cures as a classical one-component RTV. Because no water is incorporated into the silanol base component the composition does not cure as fast as those compositions where water has been specifically added.
An ultra low modulus two-component silicone sealant as disclosed by Palmer et al., U.S. Pat. No. 5,246,980, comprises a first component containing a hydroxy endblocked polydiorganosiloxane base, non-reinforcing filler, plasticizing fluid, di-functional amido-silane and an aminoxysilane oligomer. The second component comprises a hydroxy endblocked polydiorganosiloxane base, non-reinforcing filler, plasticizing fluid, and a low molecular weight hydroxy endblocked polydiorganosiloxane. The reactive amine functional silanes in the first component not only endcap the silanol polymer but also react with any water present in the formulation as adsorbed water on the filler. While this first component is shelf stable, it will cure by itself if exposed to a moist atmosphere. In contrast, the second component is inherently shelf stable and may be prepared without any particular precaution to exclude atmospheric moisture. Mixing both components together in a one to one weight ratio results in rapid deep section curing that does not require additional atmospherically supplied moisture to cure, producing an ultra low modulus silicone sealant. This particular sealant will cure to a non-flowing gel in three hours or less at 25.degree. C., reaching 35% of its ultimate cured properties in 24 hours. A related invention, U.S. Pat. No. 5,290,826 teaches the addition of water to the second component. However, the addition of water to this formulation apparently does not materially shorten the time required for the RTV to become a non-flowing gel. A significant drawback associated with the use of the di-functional amidosilane in both RTV formulations is that the modulus of the cured formulation is limited. An additional drawback associated with the use of the amidosilane is that endcapping and curing reactions release teratogenic N-alkyl amides of carboxylic acids, e.g. N-methylacetamide.
A tin compound containing composition as one of the components in a two-component system containing the essential ingredients of a triorgano substituted diorganopolysiloxane, the reaction product of a bis-silyl-alkane containing at least two monovalent hydrocarbon radicals per molecule with a diorganotin diacarboxylate and an organosilicon compound containing at least one amino or imino group per molecule is disclosed by Schiller, EP 0,612,335 B1. Additionally, a filler and/or a bis-silyl-alkane containing at least three monovalent hydrocarbon radicals per molecule which are bonded to the silicon via oxygen and are optionally substituted by an alkoxy group or an oligomer thereof.
A two-component fast-curing formulation prepared in various proportions ranging from 20:1 to 1:1 is disclosed by Mueller et al., EP 0,369,259 B1. The preferred ratio of components is 10:1 to 10:6. Component A of Mueller et al. is prepared from the reaction product of a difunctional silanol and a molar excess of an oximosilane crosslinking agent and optionally plasticizing oil, filler, dyes, catalysts, stabilizers, primers and emulsifiers; component B of Mueller et al. contains, as a minimum, hydroxyl substituted silanol polymer and water.
It is thus desirable to be able to provide a general method for the preparation of two-component RTV formulations in which each component is independently shelf stable and can be mixed with each other in a one to one weight or volume basis. It is also desirable to be able to vary the composition of both components in such a fashion that a wide variety of finished RTV products varying from extremely high strength to low modulus and hardness while maintaining a usable viscosity in the uncured components. It would also be desirable to be able to formulate the RTV components such that atmospheric moisture was not necessary to complete a rapid cure reaction, i.e. a cure achieving green strength within fifteen minutes and full cure within twenty-four hours.